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characteristic determined by the motor start-up current (Ist) and the allowable locked
rotor time (Tsc).
If the motor current is larger than 2.5 times the motor rated current, the thermal state of
the rotor can be calculated in Equation (2.28-10). Furthermore, if the thermal state of rotor
(θr) exceeds the allowable thermal capacity of rotor (θrm), the Mot-LKRT element operates.
where
θr
thermal state of rotor (percentage), showing allowable thermal capacity
θr(0)
thermal state of stator (percentage) when the rotor is locked
I(t)
motor current
Ist
motor start-up current, defined by the setting [Mot-LKRT-IS]
Tsc
allowable locking time in the cold state, defined by the setting [T-Mot-LKRT]
θrm
allowable thermal capacity of rotor as a percentage of θs (thermal state of
stator), defined by setting [Mot-LKRT]
Operating time
(ii)
If the rotor is being locked, an operating time can be varied depending on the heated
condition of the motor and the conducting current.
The below equations are provided to calculate the operating time for every condition, if motor
start-up current enters when the rotor is being locked.
Motor in cool condition
In the cool condition, θr(0) = 0 and I(t) = Ist are substituted in Equation (2.28-10).
As a results, the operating time t is Tsc (t = Tsc).
Motor in hot condition (Motor has been running at motor rated current for a
long period)
In the hot condition, θr(0) = θsn and I(t) = Ist are substituted in Equation (2.28-10).
As a results, the operating time Tsh is given by Equation (2.28-11).
where
Tsh
θsn
��(��)
θr = θr ( 0 ) + (
)
��
����
��
− ��
����
��
=
��ℎ
��
����
operating time in the hot condition
thermal state of the stator at motor rated current, given by
- 333 -
2
1
∙
∙ θ
∙ t
����
��
����
����
∙ ��
����
6F2T0207 (0.01)
(2.28-10)
(2.28-11)
GRE200 (5,6)
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